Modern image sensors necessitate three color-selective photodetectors: red, green, and blue (RGB) sensing units. Among those, blue light-specific photodetectors are comparably much less reported. Here, a fully thermal-evaporated organic photodetector (OPD) with narrowband blue detection based on exploring the dual role of the hole-transporting layer is elegantly demonstrated. By incorporating a MoO₃-doped underlayer, the narrowband OPD achieves high external quantum efficiency up to 50% at 0 V with thin-film devices. By adopting an intrinsic underlayer in planar junction configuration along with efficient hole and electron blocking layers, an ultralow dark current (2.46 × 10⁻¹² A cm⁻² at −0.1 V) is achieved. The device has a calculated specific detectivity (D^*) reaching a record-high value of 6.35 × 10¹⁴ Jones, outperforming commercial silicon photodetectors, and an ultrahigh linear dynamic range of 205 dB. This work paves the way for high-sensitivity narrowband organic photodetectors in the visible spectral region.